Lidar comprises techniques and apparatuses that combine laser-focused imaging with radar in order to calculate distances by measuring the return time for a signal sent from a sensor. In implementing a lidar system, often, the question of whether to utilize photon counting versus analog digitization arises.
Analog digitization has high linearity up to the limits of the analog to digital converter (ADC) used, but has limited dynamic range due to ADC bit resolution. Analog digitization tends to perform well in hard target situations where a full backscatter waveform is desirable, but lacks the low light sensitivity needed for most atmospheric lidar where the dynamic range of signals span several orders of magnitude.
Photon counting, on the other hand, is exceptionally versatile, as it is highly sensitive to low light levels. Given infinite time, photon counting theoretically has infinite sensitivity. However, photon counting exhibits two principle problems. First, due to time resolution requirements and atmospheric variability, a system does not have infinite time. Second, the method suffers from substantial nonlinearity in regions of high signal. Two sources contribute to this nonlinearity. For avalanche photodiodes (APDs) operating in geiger mode, the detector must “reset” after each output pulse, so the detector exhibits a fundamental dead time. For photomultiplier tubes (PMTs) and hybrid PMTAPDs (called Hybrid Photo Detectors or HPDs), nonlinearity is caused by the counting method, as multiple photons arrive within a fixed period, but only one photon can be detected at a time. In essence, coincident photons are missed either because the detector cannot quench its pulse release mechanism fast enough (e.g. in APDs) or the counter cannot register pulses that are have piled up (e.g. in PMTs and HPDs). While correction of this nonlinearity has been attempted, in practice the correction rarely works well at all photon count rates.
One problem with obtaining a linear atmospheric profile from photon counting is the substantial difference in dynamic range one can expect from low and high altitude profiles. Obtaining linear observations at low altitudes requires attenuation, which inherently works against obtaining statistically significant profiles at higher altitudes. This means that higher altitudes will require particularly long integration times. Some workarounds for this are through the use of high repetition rate and low pulse energy (commonly referred to as “micropulse”) lasers, where signal shots are unlikely to produce nonlinear response and they can be integrated on rapid temporal scales. The problem, however, is that pulse energy can be so low it cannot overcome background noise limits, while higher repetition rates limit the maximum resolvable range of the lidar.
In an effort to address single photon counting nonlinearity, detector hybrids that output both a digitized analog signal and a photon counting signal have been developed, but the user has to combine or “stitch” together the two signals. This is problematic because one needs an overlap area where the ADC has sufficient resolution to accurately represent the signal (which is best suited to large signals) and the photon counting system is linear (which is best suited to small signals). The area of overlap where the photon counting system is linear and the analog system is low noise tends to be small if even present at all.
The embodiments described below overcome these and other problems and an advance in the art is achieved. The embodiments described below provide a hybrid technique between photon counting and analog digitization that offers the high-end dynamic range of an analog digitizer combined with the low-end dynamic range of a photon counting system. The method and apparatus utilize a fully integrated approach, thus avoiding problematic signal stitching. The embodiments essentially digitize PMT (or HPD) pulse waveforms instead of analog backscatter waveforms. The pulses are integrated so their energy is captured rather than relying on a binary pulse count. Therefore, the embodiments do not require the signal to fall below a pulse count threshold before additional signals are registered, so pulse pileup issues are avoided.